Image display devices provided with backlights, such as liquid crystal display devices, can control the luminances of the backlights on the basis of input images, thereby suppressing power consumption by the backlights and improving the quality of display images. In particular, by dividing a screen into a plurality of areas and controlling the luminances of backlight sources corresponding to the areas on the basis of portions of an input image within the areas, it is rendered possible to achieve lower power consumption and higher image quality. Hereinafter, such a method for driving a display panel while controlling the luminances of backlight sources on the basis of input image portions within areas will be referred to as “area-active drive”.
Liquid crystal display devices of area-active drive type use, for example, LEDs (light emitting diodes) of three colors, i.e., R, G and B, and LEDs of white as backlight sources. Luminances of LEDs corresponding to areas are obtained on the basis of, for example, maximum or mean pixel luminances within the areas, and provided to a backlight driver circuit as LED data. In addition, display data (data for controlling the light transmittance of the liquid crystal) is generated on the basis of the LED data and an input image, and the display data is provided to a liquid crystal panel driver circuit. Note that the luminance of each pixel on the screen is the product of the luminance of light from the backlight and the light transmittance based on the display data. Here, light emitted by one LED is incident on a plurality of areas around one corresponding area. Accordingly, the luminance of each pixel is the product of the total luminance of light emitted by a plurality of LEDs and the light transmittance based on the display data.
In the liquid crystal display devices as mentioned above, the display data and the LED data are appropriately obtained on the basis of an input image, the light transmittance of the liquid crystal is controlled on the basis of the display data, and luminances of LEDs corresponding to areas are controlled on the basis of the LED data, so that the input image can be displayed on the liquid crystal panel. In addition, when luminances of pixels within an area are low, luminances of LEDs corresponding to that area are kept low, thereby reducing power consumption by the backlight.
In relevance to such area-active drive liquid crystal display devices, for example, International Publication WO2009/096068 pamphlet discloses the configuration of a conventional image display device (hereinafter, the device will be referred to as the “conventional device”) in which upper and lower luminance limits are predetermined for use in obtaining light source luminances corresponding to areas. In the conventional device, in accordance with a mean image luminance, the upper luminance limit is determined to be lower than the maximum luminance, or the lower luminance limit is determined to be lower than the minimum luminance, thereby preventing a reduction in display quality.
Patent Document 1: International Publication WO2009/096068 pamphlet